Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 7,842,667	$ 7,862,311
Unrealized Gains	103,101	12,082
Unrealized Losses	(5,446)	(84,350)
Carrying Value	7,940,322	7,790,043
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,757,164	4,762,182
Unrealized Gains	69,279	11,638
Unrealized Losses	(5,369)	(35,562)
Carrying Value	4,821,074	4,738,258
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,090,426
Unrealized Gains		61
Unrealized Losses		(6,370)
Carrying Value		1,084,117
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,203	5,815
Unrealized Gains	8	0
Unrealized Losses	0	(3)
Carrying Value	9,211	5,812
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,303,634
Unrealized Gains	18,245
Unrealized Losses	(22)
Carrying Value	1,321,857
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,772,666	1,922,618
Unrealized Gains	15,569	0
Unrealized Losses	(55)	(42,400)
Carrying Value	$ 1,788,180	1,880,218
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—variable rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		81,270
Unrealized Gains		383
Unrealized Losses		(15)
Carrying Value		$ 81,638

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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